It is widely known in the art to use electrical induction devices (e.g. power transformers), which exploit the electromagnetic induction for properly transmitting and distributing electricity over power lines.
Known power transformers include live parts, such as a magnetic core and a certain number of windings, for instance low-voltage windings, high-voltage windings, et cetera.
Due to the intrinsic structural characteristics and functioning of these devices, important aspects of power transformers concern the electric insulation among the various components and cooling thereof which will be guaranteed to provide the desired electromagnetic performance without incurring any malfunctioning or damage.
To this end, a power transformer can include a closed main tank which is filled with an insulating fluid and which houses the live parts; the insulating fluid can be a liquid, for example a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties; combustion-resistant vegetable oil-based dielectric coolants are also becoming increasingly common as alternatives to mineral oils.
In addition, power transformers can be provided with expansion vessels indicated as oil conservators; such conservators are positioned above the main tank, and have the function of compensating the unavoidable volume changes of the cooling fluid used in the tank, which volume changes result mainly from temperature fluctuations.
Since the insulating liquid helps cooling of the transformer and also contributes to the electrical insulation between live parts inside the tank, it should remain stable at high temperatures for an extended period.
During the working life of a power transformer, it is possible that gas is generated or present inside the tank and this is a clear indication of a possible problem.
For example, the gas may be the result of decomposition/degradation of the solid or liquid insulation inside the transformer caused by overheating or by the strike of electric arcs, or the gas may come from the insulating oil itself due to unsatisfactory de-gassing prior to filling the tank.
In addition, rapid movements, also indicated as rapid currents or flows, of the transformer liquid can be caused by an internal arc, short circuit, or hot spot; these rapid movements are indicative of possible abnormal or dangerous conditions and must be properly addressed.
In order to cope with these issues, there are used suitable safety devices, commonly indicated in the art as Buchholz relays, so that the generation of gas and the presence of rapid movements are detected and related risks are prevented or mitigated as much as possible.
An exemplary Buchholz relay has a chamber inside which the gas formed or present in the transformer is accumulated, as defined by rules and standards, for instance EN 50216.
With the aim of monitoring the quantity of gas formed, Buchholz relays are equipped with two floats which are hinged onto a supporting frame, are immersed into the liquid of the transformer, and are operatively associated to corresponding switches.
In detail one float, which is positioned inside the chamber at an upper part, starts to rotate at the onset of gas formation and activates some electrical contacts so as to cause an alarm signal if the quantity of gas is such that the level of liquid inside the chamber reaches a first alarm threshold. The second float starts to rotate after the alarm signal is generated and activates some corresponding electrical contacts to trip the transformer and disconnect it from power feeding when the quantity of gas is such that the level of liquid inside the chamber reaches a second trip threshold.
Although these known safety devices acceptably perform their functions, there are some aspects that can be optimized, such as with regard to the constructive layout of and the space available inside the safety device itself. Indeed, there are more and more requirements to add new functionalities to the existing safety devices, such as for example to be able to continuously measure the gas formation and to properly monitor the transformer functioning over time and even remotely. With the actual constructive layout, the space inside a traditional Buchholz relay may not be optimal and the actual floating equipment can be quite cumbersome.